AGA5802 Spectroscopy II Prism Gratings Applications

Transcription

1 AGA5802 Spectroscopy II Prism Gratings Applications Bibliography: To Measure the Sky, Kitchin, Lena and others... Prof. Jorge Meléndez 1

2 Slit Basic components of the Spectrograph Prism or grating Roy & Clarke 2

3 Dispersing element Diffraction Grating: diffraction+interference Prism: differential refraction 3

4 Homework #5: build an spectrograph based on a CD or DVD grating : deadline May 4th 07/issue4/spectrometer pt/2007/issue4/spectrometer stro/html/spectrometer.html 4

5 Emission lines in compact fluorescent lamp using spectrograph built by undergrad student of Stellar Astrophysics USP

6 Prism as a dispersing element q exit? n air sin q air = n glass sin q glass q exit n(air) = Kitchin 6

7 Dispersing element: prism 7

8 Snell law in a prism 1 (or n 1 ) is ~ 1 (air), so : n 1 sin i 1 = n 2 sin r 1 means that n 2 = sin i 1 / sin r 1 In the same way : n 2 sin i 2 = n 1 sin r 2 n 2 = sin r 2 / sin i 2 a q r 2 i 1 r 1 i 2 Kitchin 8

9 Refractive index ( l or n l ) varies with wavelength In the visible the variation of n l may be approximated by the Hartmann dispersion formula. A, B and C are known as the Hartmann constants Kitchin 9

10 10

11 a For a given l, the deviation angle is q q r i 1 + a = 180 a q a q r 2 i 1 r 1 i 2 a + 90 r i 2 = Kitchin

12 For a given l, the deviation angle is q a sin r 2 = l sin i 2 i 2 = a r 1 sin r 1 = sin i 1 / l q r 2 i 1 r 1 i 2 r 2 12 Kitchin

13 Maximizing dispersion Dq/Dl Example for dense flint prism Kitchin 13

14 Dq/Dl ( 0 m -1 ) Variation of Dq/Dl with angle of incidence i 1 for different a i 1 a q x10 7 a = 50 73, ,7 73 a = 70 Maximum dispersion of 1,02 x m -1 occurs for i 1 = 90 0 and an apex angle of 73, a = i 1 14

15 Maximizing dispersion Dq/Dl symmetrical passage of the ray through the prism maximizes dispersion and minimizes optical aberrations 15

16 symmetrical passage is also called position of minimum deviation (of q) Example: deviation q for a dense flint prism with apex angle a = 30 O at l = 500 nm q a i 1 r 2 r 1 i 2 minimum deviation At min dev, r 1 = i 2 = a/2 i 1 16

17 a i 1 r 2 r 1 i 2 At min deviation r 1 = i 2 = a/2 symmetrical passage 17

18 Compromise, a = 60 O a r 1 =? dispersion of a prism increases rapidly towards shorter l 18

19 Spectrograph based on prism CCD 19

20 Objective prism spectrograph 20

21 Classic applications of spectrographs 21

22 The beginning of stellar spectroscopy 1802: William Wollaston first observed dark lines in the solar spectrum which he incorrectly interpreted as gaps separating the colors 1817: Joseph Fraunhofer (1817) observed a continuous color change without color discontinuities at the dark lines. He found 574 lines. 1836: Sir David Brewster found that certain lines had strengths that varied with the Sun's elevation. He correctly ascribed these as originating in the terrestrial atmosphere. O 2 (B) O 2 (A) Ca II (K) (H) Ca(g) 4227 H (h) 4102 Fe(e) 4384 H (f) 4340 CH, Fe (G) 4308 Fe (d) 4668 H Fe (F) (E) Mg (b 1, b 2 ) 5184 & 5173 Fe (c) 4958 Na 5890 & 5896 D 2 D 1 Ha (C) 6563

23 Stellar classification Secchi's Classes of Stellar Spectra ( ) Secchi's four classes of stellar spectra, from a colored lithograph in a book published around The principal spectral lines are identified underneath by letters that Fraunhofer assigned. Type I: white-blue; strong H lines. Current class A & early F Type II: yellow, tipo solare. Numerous metallic lines (Na, Ca, Fe), with weaker H. Current class G, K, late F Type III: orangered; metallic lines and bands. Current class M Type IV: stars with emission lines

24 The Harvard classification system s: Harvard classification (E.Pickering + Williamina Fleming + Antonia Maury + Annie J. Cannon): O, B, A, F, G, K, M Women Harvard

25 Stellar classification: O, B, A, F, G, K, M Based on spectra taken at the Harvard North (USA) & South (Arequipa, Peru) stations Annie J. Cannon classified > spectra!

26 The Henry Draper (HD) catalogue Ultimately listed over stars. It is still very useful; the most common ID of stars with V < 9 is its HD number (also HIP) Harvard plate taken with objective prism spectrograph in Arequipa. Field of η Carinae. E. Dorrit Hoffleit, 2002, Phys. Perspect., 4, 370

27 Luminosity class Antonia Maury : hired in 1888 by E. Pickering (Harvard) to classify spectra. She proposed a new system considering also the width of the lines, but was ignored Pickering. Dwarf Supergiant

28 Wavelength (nm)